The present invention relates generally to so-called beaded retroreflective sheeting. More particularly, the invention relates to such sheeting that, in addition to being retroreflective, is also luminous in that it emits self-generated light. The self-generated light is preferably electroluminescent ("EL") in nature.
The reader is directed to the glossary at the end of the specification for guidance on the meaning of certain terms used herein.
Retroreflective sheeting can be used in a variety of end uses, such as highway signs and markers, license plates, advertising boards, truck conspicuity markers, clothing, security applications, and the like. Most such sheeting can generally be classified as cube corner sheeting or beaded sheeting, according to the structure responsible for retroreflection: in cube corner-type sheeting, faces formed on a structured surface are arranged to form elements in which three neighboring faces are approximately mutually perpendicular so as to produce retroreflection; in beaded-type sheeting, a multitude of miniature transparent beads focus incident light onto a reflective surface, the reflected light then passing back through such beads towards the source of the incident light. The present application is directed to the broad category of beaded sheeting, which includes a wide variety of known constructions.
In some constructions, referred to as enclosed lens constructions, the beads are substantially fully embedded in one or more polymeric layers. In these constructions, retroreflectivity of the sheeting will not be destroyed if a film of water forms on top of such layers. See, e.g., U.S. Pat. No. 2,407,680 (Palmquist et al.), U.S. Pat. No. 4,367,920 (Tung et al.). In other constructions, referred to as exposed lens constructions, the beads are partially embedded in a polymeric layer and partially exposed to air or vacuum. The retroreflectivity of these latter constructions can be eliminated or greatly diminished if a film of water were to form on the exposed portions of the beads. Therefore, many exposed lens constructions include an additional cover layer to encapsulate the exposed beads in sealed cells. See, e.g., U.S. Pat. No. 3,190,178 (McKenzie), U.S. Pat. No. 4,025,159 (McGrath).
Various attempts have been made to provide beaded retroreflective sheeting with luminous characteristics. These attempts have various drawbacks associated with the separate manner in which the luminous agents and the retroreflective agents are incorporated into the article. For example, in JP-A-60-205501 (Hiroshi et al.), a bead layer comprises a resin film containing beads as well as small particles of light accumulative pigment. Below the bead layer is a reflective layer comprising a resin film containing a fluorescent pigment and mica titanium. The relatively large beads and relatively small light accumulative pigment particles are dispersed in the resin film that forms the bead layer. This layer is formed by coating and drying the dispersion, and in practice almost all the surfaces of the beads would become covered with the light accumulative particles, tending to degrade retroreflectivity. It then becomes necessary to reduce the amount of light accumulative particles enough to keep the retroreflectivity at an acceptable level. The resulting tradeoff between retroreflectivity and luminescent brightness is inefficient because of the fashion in which the beads, the light accumulative pigment, and the luminescent particles are distributed. Fabrication is also complicated because of difficulties dispersing small particles uniformly in a matrix resin solution, such particles tending to sink or float in a layer of such solution prior to hardening.
Luminous retroreflective sheetings would benefit from more efficient distribution of retroreflective and luminous agents. Desired sheeting constructions would require only slight modifications to existing manufacturing processes used to make purely retroreflective sheetings, and would thus be compatible with relatively simple production steps and low production costs.